1. Field of the Invention
The present invention relates to improvements in a patented energy storage method for use with cranes and other load hoisting machinery. More particularly, it relates to improvements in a method for storing energy in a capacitor which is charged by the load hoisting machinery drive motor. The stored energy is resupplied to the system when the load is being raised and needs more power.
2. Description of the Prior Art
The present invention relates to improvements in a system or method for energy storage in load hoisting cranes which are driven by electrical power. It is particularly useful for machinery which is driven by diesel-electric generators that experience a wide range of varying loads. The system stores energy at reverse or small load and supplies power at peak or large loads. Theoretically, this is a simple mechanical query, having as a result the benefit that the primary electrical source is only required to supply relatively constant average power and is not required to supply peak power. However, until now, the practical aspects of the query have prevented its use.
Combination battery and generator energy storage systems have been utilized to accomplish this result in the past, and theoretically they are very effective. However, in reality, the battery component imposes numerous problems such as: small electrical capacity, electrical inefficiency, large physical battery volume, heavy weight, and short battery life, whereby such a system is not currently a viable way to accomplish energy storage utilizing even state-of-the-art battery technology.
Flywheel type energy storage systems have also been utilized to accomplish the result. However, in order for the flywheel to store energy to create power, it must be capable of being driven over a wide range of speeds. In order to transmit the energy to the flywheel at the variable speeds, a DC motor has been utilized as most suitable, but the DC motor-driven flywheel has not been proven satisfactory for numerous reasons among which the following are most limiting:
1. In order for the flywheel to store energy, the energy is measured by ½×I×ω2 where I=the moment of inertia, and ω=the rotating angular speed. Therefore, high rotating speeds can store much more energy in the flywheel because the energy is measured by a square of the rotational speed. However, the DC motor which must be interconnected to the flywheel has severe rotational speed limitations due to the weak centrifugal strength of its rotor's coil component;
2. The DC motor requires continuous maintenance such as brush replacement, commutator repair, and maintaining insulation integrity;
3. A DC motor is comparatively large, heavy, and expensive.
For these reasons and others, the flywheel-driven energy storage type system utilizing a DC motor has likewise not been a viable way to accomplish the result.
Recent developments in inverter technology have progressed to the point where AC squirrel cage induction motors using inverters are replacing DC motors. The inverter converts DC to AC with arbitrary frequency and also converts AC to DC in reverse. By virtue of the AC arbitrary frequency, the AC squirrel cage induction motor can rotate with arbitrary rotational speed up to very high speeds solving some of the described problems associated with DC motors.
FIG. 1 of the drawings shows a typical example of currently utilized diesel-generator power sources and inverter controlled induction motor drive machinery for load hoisting machinery. The diesel engine 11 is mechanically interconnected to an AC generator 13. The alternating current output from the generator is converted to direct current by a diode 15. The DC, in turn, is converted to AC with an arbitrary frequency by the inverter 17. A squirrel cage induction motor 19 is driven by the AC and, in turn, drives a drum 21 which raises or lowers a load 23. The raising and lowering speeds are controlled as a result of the alternating current frequency generated and controlled by the inverter. When the load is lowered, reverse AC current is generated by the induction motor. The reverse current is consumed by a resistor 25 in order for the induction motor to operate effectively as degenerative braking.
FIG. 2 of the drawings discloses a typical example of current from a municipal utility power grid 27 being fed to the system by a cable reel power supply 29 instead of from the diesel engine/generator combination of FIG. 1. The incoming voltage is lowered by a transformer 31. The alternating current is then converted to DC by a DC converter 16 and, from that point on, the system is the same as disclosed in FIG. 1 of the drawings.
During lowering of the load 23, reverse current is sent back to the power grid 27 and, in this example, is used by other consumers. However, since the reverse power current includes surge and deviant frequencies, other consumers dislike receiving it. It is expected that in the future sending reverse power back to the power grid may be prohibited. In that event, the reverse power will be consumed by a resistor, the same as disclosed in the system of FIG. 1.
FIG. 3 of the drawings discloses the improvement on the prior art which is inserted into the system in place of the resistor as utilized in FIG. 1 of the prior art systems. It is disclosed in U.S. Pat. No. 5,936,375, issued Aug. 10, 1999, for a Method for Energy Storage for Load Hoisting Machinery. The present invention includes further non-obvious improvements on that design.